Control arrangement with a pulse-length modulator for a piston

ABSTRACT

A servo loop for correcting the position of a piston operating against a load has a control unit comprising a three point regulator for operating two on-off valves and a pulse length modulator for operating a switch over valve, the on-off and switch over valves being connected to the piston cylinder, a pressure tank, and a collection tank, so as to move the piston in either of two directions. The three point regulator and the pulse length modulator are driven by the difference between the desired signal and the actual signal, the latter being obtained from an inductive displacement pickup coupled to the piston rod.

United States I Patent [1 1 Fournell et al.

[ CONTROL ARRANGEMENT WITH A PULSE-LENGTH MODULATOR FOR A 4 PISTON Oct.9, 3,295,421 1/1967 McCormick ..9l/363R 3,618,469 11/1971 Willsetal...9l/417R [75 I Inventors: zanlsgi eterAiiitiurnlell, BLetigheim;Primary Examiner paul Maslousky ar emz erg; Attome -Michael S. StrikerJohannes Locher, Stuttgart, all of y Germany [73] Assignee: Robert BoschGmbH, Stuttgart,

Germany [57] ABSTRACT [22] Filed: Mar. 11,1971

A servo loop for correcting the position of a plston op- [211 Appl'128,607 erating against a load has a control unit comprising a threepoint regulator for operating two on-off valves [30 Foreign ApplicationPriority Data and a pulse length modulator for operating a switch Mar 121970 German P 2O 11 713 9 over valve, the on-off and switch over valvesbemg y connected to the piston cylinder, a pressure tank, and acollection tank, so as to move the piston in either of [52] US. Cl91/20,9911/g3l27, 99110346530, two directions The three point regulatorand the pulse [51] In Cl Flsb 15/17 Flsb 9/03 FISb 9/09 length modulatorare driven by the difference between [58] Fie'ld 91/363 i 457 275 thedesired signal and the actual signal, the latter 91/417 being obtainedfrom an inductive displacement pickup coupled to the piston rod. [56]References Cited U E STATES PATENTS 16 Claims, 5 Drawing Figures3,266,378 8/1966 Shaw 91/363 R t, I I 20 I 21 l1 PRESSURE r 7 PRESSREGULATING I QE Q PRESSURE VALVE MV3 5 l9 REGULATING l VALVE l f i f 1 L1 1 MV2 I MV1 7 r 19 17 2 l .12 J I I I 13 15 i l 18 25 j m I l 16 i Il4 1h PATENTEBBCT ems SHEEI 10F 2 21 PRESSURE REGULATING VALVE PRESSUREREGULATING VALVE FIG] PRESSURE REGULATING VALVE INVENTORSHons-DieterFOURNELL. Karl Heinz ADLER Johannes LOCHER Lg ELMVE:

their ATTORNEY PATENTEDUCT 9 I973 Q B F SHEET 2 BF 2 INVENTORSHons-Dieter FOUR N ELL Karl-Heinz ADLER Johannes LOCHER their ATTORNEYCONTROL ARRANGEMENT WITH A PULSE-LENGTH MODULATOR FOR A PISTONBACKGROUND OF THE INVENTION The invention relates to a hydraulic controlarrangement having a piston and cylinder with an inlet chamber and anoutlet chamber. Electromagnetic valves, operated by a control unit,control flow of hydraulic fluid from a pressure tank and to a collectiontank.

Hydraulically operated distributors for gas turbines must be positionedquickly, very accurately, and with great force. The same demands aremade of the hydraulic fine control elments that position the adjustingrod of a diesel fuel injection pump controlled by an electronicregulator.

Servo valves are used for precision control arrangements, these valvesconverting a continuously varying electrical signal into a proportionalflow of hydraulic fluid. These valves can control the movement of apiston very precisely, but they are expensive, precision made, products,and for that reason not suitable for mass produced equipment. Moreover,as precision components, they are sensitive to contamination of thehydraulic fluid and to strong vibrations, among other factors, andtherefore are not suitable for operation under severe conditions. I

SUMMARY OF THE INVENTION An object of the invention is a simple andinexpensive control arrangement for a hydraulic piston.

Another object of the invention is the arrangement of the precedingobject suitable for mass production.

A still further object of the invention is a control arrangement for ahydraulic piston, which control arrangement operates quickly andaccurately.

A further object of the invention is a control arrangement of theprevious object, which control arrange ment positions the piston with anaccuracy of at least 1 percent of the desired position.

Another object of the invention is a control arrangement for ahydraulicpiston, which control arrangement is suitable for operationunder severe conditions, such as those found ina motor vehicle.

Briefly stated, the invention consists of piston means, pressure tankmeans, collection tank means, electromagnetic valve means foralternately connecting the pistonmeans to the pressure tank means and tothe collection tank means so as to cause movement of the piston means ineither of two directions, and control means, including pulse lengthmodulation means, for providing electrical signals to operate the valvemeans so as to cause movement of the piston means toward the desiredposition corresponding to a desired value, the pulse length modulationmeans having a predetermined keying ratio when the piston means is atthe desired position.

,The novel features which are considered as characteristic for theinvention are set forth in particular in the appended claims. Theinvention itself, however, both as to its construction and its method ofoperation, together with additional objects and advantages thereof, willbe best understood from the following description of specificembodiments when read in connection with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of oneembodiment of the invention;

FIG. 2 is a block diagram of the control unit;

FIG. 3 is a wiring diagram of the pulse length modulator of the controlunit;

FIG. 4 is a wiring diagram of the three point regulator of the controlunit; and

FIG. 4a graphically shows the output of the operational amplifierplotted against the input voltage.

DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference to FIG. 1, thehydraulic control arrangement of the invention includes first valvemeans including two one-way electromagnetic first valves MVl and MV2 anda parallel connected electromagnetic two-way, or switch over, secondvalve means MV3. A pressure regulating valve 11 connects the valve MVl'to a pressure tank 10, and a line 12 connects this valve to the inletof a hydraulic cylinder and piston arrangement 13. The inlet of thearrangement 13 is defined as being that opening through which hydraulicfluid enters the piston chamber (the inflow chamber) to act on thepiston face opposite the piston rod 16, whereas the outlet is defined asthat opening which communicates with the piston chamber (the outflowchamber) containing the piston rod 16. The hydraulic arrangement 13comprises a cylinder 14, a piston 15, and a piston rod 16. The valuesMVl and MV2 control the fast movement of the piston 15 to approximatelythe piston position corresponding to the desired value. The valve MV3 isa fine control for the piston movement in the immediate neighborhood ofthis position. A line 17 connects the piston outlet to the pressure tank10. The electromagnetic valve MV2 is connected to the line 12 and by aline 19 to a collection tank 18. A pressure regulating valve 20 connectsthe line 17 to one connection of the electromagnetic switch over valveMV3, the other connection of this valve being connected by a pressureregulating valve 21 to the line 19. The electromagnetic first valvemeans MVI, MV2, and second valve means MV3 are operated by a commoncontrol unit 22, the electrical connections between the latter and thesevalves being denoted by the dashed lines. The control unit 22 isfurnished with a desired value by an electrical signal generator 23,which will not be further described. The control unit is also suppliedwith an actual value by an electric signal generator 24, which iscoupled to the piston rod 16. The arrow X indicates the direction inwhich the piston rod moves when it works against a load denotedsymbolically by an arrow 25.

The valves MVl and MV2 operate as on-off valves, the amount of hydraulicfluid passing through them being limited solely by the pressureregulating valve 11. Since the valve MV3 operates as a switch overvalve, one of the two lines 17 and 19 is always connected by therespective pressure regulating valve 20 or 21 to the line 12. If thepiston 15 is to move in the direction of the arrow X, the valve MVl isfirst of all opened and the valve MV2 is closed. The hydraulic fluid,which is under pressure, is thus free to flow through the line 12 andinto the inflow chamber of the piston 13, as well as through the line 17and into the outflow chamber of the piston. Since the face of the piston15 in the inflow chamber is larger than the opposite piston face by anamount equal to the cross-sectional area of the piston rod 16, thepiston 15 is caused to move to the left against the load 25. If thepiston is to move in the opposite direction, the valve MV1 is closed andthe valve MV2 is opened. A force of equal magnitude in either directionof piston movement is obtained when the effective area of the two facesof the piston 15 have a ratio of 2 1. This means that the effective areaof the piston face in the outflow chamber is equal to the crosssectionalarea of the piston rod 16. When valve MVl is open and valve MV2 isclosed, the pressure of the hydraulic fluid acts through the line 17 andon the piston face in the outflow chamber. Because of the ratio betweenthe effective surface areas of the two piston faces, the piston 15 iscaused to move in the direction of the arrow X, even though both pistonfaces are under the same pressure. As soon as the piston rod 16 is movedto near the desired position (corresponding to the desired valuesupplied by the desired signal generator 23), the electromagnetic valveMV1 is closed; and the electromagnetic valve MV3 now assumes the role ofprecisely positioning the piston. The valve MV3 is operated by a pulselength modulated signal from the control unit 22. if the deviationbetween the desired value and the actual value (in other words, betweenthe desired position and the actual position of the piston) is stillrelatively large, the periods during which the line 12 is connected tothe line 17 are appreciably longer than the periods during which theline12 is connected to the line 19.

Only when the deviation is zero are these two periods of equal length,the piston rod 16 oscillating back and forth about the desired position.Care must be taken that the frequency at which the valve MV3 iscontrolled is so chosen that this valve is able to follow exactly thesignals from the control unit 22, and that it does not, because of itsinertia, operate imprecisely.

The signal generator 24 is advantageously an inductive displacementpickup, since this design avoids sliding contacts and substantiallyimproves reliability.

The manner in which the illustrated exemplary embodiment operates is asfollows.

Assume that piston 15 is in the illustrated position, and that thedesired position, set by unit 13, is leftwards of the illustratedposition by a considerable amount, requiring a sizable initial coarseadjustment of the piston position. Control means 22 senses the need foran initial coarse adjustment, by determining that the difference betweenthe actual and desired piston positions is greater than a predeterminedvalue, or equivalently, outside a fine-adjustment range centered aboutzero. Control means 22 also determines the algebraic sign of thedifference, indicating the direction in which the piston is to be moved.

Assuming that piston 15 is to be moved leftwards, control means 22merely keeps valve MV2 closed, and opens valve MVl, valves MVl and MV2having both been closed prior to this moment in the steady-statecondition of the arrangement. Two-position valve MV3 is so actuated asto permit fluid flow to conduit 12 from the shunting conduit includingvalve 20, and thus preventing fluid flow between conduit 12 and theshunting conduit section which includes valve 21.

With the valves so actuated, pressure fluid flows from source throughboth conduit sections 17, i.e., directly to the left-hand cylinderchamber, and also to the right-hand cylinder chamber via valve MV3 andconduit 12, on the one hand, and via valve MVl and conduit 12, on theother hand. Accordingly the left-hand and right-hand cylinder chamberswill be at identical pressure. However, inasmuch as the effective areaof the right-hand piston end face is twice that of the lefthand pistonend face, the leftwards force will be twice the rightwards force, andthe piston will be moved leftwards. This initial coarse adjustment willoccur relatively fast.

Quickly, the difference between the actual and desired values of pistonposition will decrease below a predetermined value, indicating that thecoarse adjustment is to end and that the supplementary fineadjustment ofpiston position is to commence At this point, control means 22 closesvalve MV], and further movement of the piston towards the desiredposition is effected solely by actuation of two-path valve MV3, in thisembodiment.

As indicated schematically in FIG. 1, valve MV3 is normally springbiassed to permit fluid communication between upper conduit section 12and the shunting conduit containing valve 21. However, when controlmeans 22 supplies a current pulse to the magnetic winding of two-pathvalve MV3, valve MV3 will terminate the communication between conduit 12and conduit l9, and will effect communication between conduit 12 andconduit 17for the'duration of such current pulse.

Control means 22 includes pulse-length-modulating means, to be describedin detail below. The pulselength-modulating means generates a train ofpulselength-modulated pulses, according to well known principles. In theillustrated embodiment, the keying ratioi.e., the ratio of thepulse-duration to the time interval between successive pulses-varies asa function of the difference between the desired and actual pistonpositions, and as a function of the algebraic sign of such difference.

The effect of valve MV3 on piston movement will be self-evident. Whenvalve MV3, in response to electromagnetic actuation, permits fluidcommunication between conduits 12 and 17 (valve MVl has already beenclosed), pressure on both sides of piston 15 will be identical. Becausethe surface area of the right piston end face is greater than that ofthe left end face, piston 15 will be moved leftwards. Conversely, whenin the absence of an actuating current pulse, MV3 permits fluidcommunication only between conduits l2 and 19, piston 15 is movedrightwards. This is because pressure fluid from source 10 enters theleft-hand cylinder chamber, whereas pressure fluid in the right-handcylinder chamber flows out via conduit 12, valve MV3 and conduit 19 tofluid collection tank 18.

It will be appreciated that the direction of piston movement isdetermined by the aforementioned keying ratio of thepulse-length-modulated pulse train, during the fine-adjustment phase ofpiston movement. If the piston is to move leftwards in the illustratedembodiment, the pulse-duration must be longer than the time intervalbetween successive pulses. If the piston is to move rightwards in theillustrated embodiment, then the pulse-duration must be shorter than thetime interval between successive pulses. The keying ratio is dependenton the difference between the desired and actual piston positions, andwhen the difference approaches zero, the keying ratio approaches a fixedvalue which will cause the piston, inthe steadystate, to oscillatesteadily about the desired value, very slightly.

It should be noted that the surface area .of the righthand piston endface is twice that of the left-hand piston end face. It follows,therefore, that the leftwards force to which the piston is subjectedwhen both cylinder chambers are under pressure is exactly equal inmagnitude to the rightwards force to which the piston is subjected whenonly the left-hand cylinder chamber is under pressure. Accordingly,there is force symmetry in this arrangement. Furthermore, once thedesired piston position has been reached, the keying ratio will become 1l, i.e., the pulse-duration will be equal to the time interval betweensuccessive pulses, and the piston will oscillate steadily, although onlyslightly, about the desired position.

FIG. 2 is a block diagram of the control unit 22. The inputs 30 and 31are respectively connected to the actual value signal and the desiredvalue signal. The control unit 22 comprises valve-actuating means, herea three point regulator 32 and a quasi-stabilized pulse length modulator33, the input of which latter is connected in parallel with the input ofthe regulator 32. The two electromagnetic valves MVl and MV2 areconnected to the output of the regulator 32. The output of the regulator32 shifts between first and second limiting values, in one of which thevalve MVl is open and in the other of which the valve MV2 is open. Inthe dead zone defined between these first and second limiting valuesboth of the valves MVl and MV2 are closed. The switch over valve MV3 isconnected to the pulse length modulator 33. The difference between theactual value and the desired value is formed by the voltage drops acrossresistors, the polarity of the actual value and of the desired valuenecessarily being so chosen that the desired deviation between these twovalues appears at the summation junction. A resistor 34 and a parallelconnected capacitor 35 connect the input terminal 30 to a summationjunction 46 at the input of the three point regulator 32; a resistor 37connects the other input terminal 31 to this same junction. Theresistors 38 and 39 respectively connect the inputs 30 and 31 to asecond'summation junction 40 that is connected to the input of the pulselength modulator 33.

The circuit thus far described operates in the following manner. If thedeviation between the desired signal and the actual signal exceeds apredetermined value, the output of the regulator 32, in dependence onthe polarity of the deviation, swings to one or the other of its firstand second limiting values, thereby operating one of the twoelectromagnetic valves MVl and MV2. If the loading is small, it canoccur that the actual value changes so rapidly that the dead zone hasbeen traversed before the valve can respond. This disadvantage isavoided by the capacitor C35, which provides an approximate first timederivative of the actual value that is added to the signal at thesummation junction 36. This additional signal causes an apparentincrease in the size of dead zone of the regulator 32, and ensures thatthe dead zone will not be overshot before the valve can respond. Whilethe valve MVI or MV2 is open, the valve MV3 can also be in one of itstwo positions, which it assumes when the pulse length modulator is overdriven. The region in which the keying ratio of the pulse lengthmodulator changes approximately coincides with the region of the deadzone of the regulator 32. The operation of the valves MVl and MV2 isassisted by the state of the valve MV3. As soon as the pulse lengthmodulator is in its linear region, it operates with a keying ratio thatchanges in proportion to the deviation until the desired value isreached, the deviation then being zero, and the keying ratio being somepredetermined value, such as l I.

If the control unit 22 contained only a three point regulator, thepositioning accuracy would be limited by the dead zone of the regulator;and the switching frequency would result from the time responsecharacteristic of the servo loop. I

If the control unit 22 had only a two point regulator, the piston wouldbe caused to oscillate markedly back and forth across the desiredposition, since the valves controlled by the regulator would be usedboth for high speed positioning of the piston as well as for holding thepiston at the desired position.

With the arrangement of the invention there is also a continuousoscillation about the desired value; but the flow of hydraulic fluidthrough the valve M3 can be sufficiently limited, and the controlfrequency of the pulse length modulator 33 can be chosen sufficientlyhigh, so that the path over which the piston oscillates remainssufficiently small to insure the desired accuracy of better than orequal to 1 percent. The maximum flow through the valves MV], MV2, andMV3 is determined by the pressure regulating valves ll, 20, and 21. In aparticularly advantageous embodiment of the invention, the crosssectional areas of the valves MVl, MV2, and MV3 can be dimensioned toact as pressure regulating valves.

FIG. 3 is a schematic diagram of one embodiment of the quasi-stabilizedpulse length modulator .33, the modulator incorporating an operationalamplifier 45. The amplifier 45 has two inputs, an inverting input M anda non-inverting input P; the input circuit of the operational amplifiercan comprise, for example, a differential amplifier. The non-invertinginput is connected to the tap of a voltage divider composed of theresistors R46 and R47, the resistor R46 being connected to the positiverail 48 connected to a source of voltage Ub and the resistor 47 beingconnected to the grounded rail 49. An adjustable load resistor R50connects the output of the operational amplifier to the inverting input,and a positive feedback resistor R51 connects the output of theoperational amplifier to the non-inverting input. The operationalamplifier is connected to the rails 48 and 49 for power. The inputterminal 31 (desired value) is connected to the tap of an adjustableresistor R52, which is connected between the rails 48 and 49. The signalof the desired value at the tap of the resistor R52 is conducted byresistor R39 to the summation junction 40, which is connected by anadjustable capacitor C to the grounded rail 49. The electrical output ofthe operational amplifier 45 is conducted by a three stage poweramplifier, comprising the transistors T53, T54, and T55, to theelectromagnetic valve MV3.

The base of transistor T53 is connected by a resistor R56 to the outputof the operational amplifier 45 and by a resistor R57 to the groundedrail 49. The collector of the transistor T53 is connected by a resistorR58 to the positive rail 48, the emitter of this transmitter beingdirectly connected to the base of the transistor T54, the emitter ofwhich is directly connected to the grounded rail 49 and the collector ofwhich is connected by a re-' sistor R59 to the positive rail 48. The twotransistors T53 and T54 comprise a Darlington amplifier. A resistor R60connects the collector of the transistor T54 to the base of transistorT55, the emitter of which latter is connected by a diode D61, connectedwith forward polarity, to the positive rail 48. The emitter collectorpath of this transistor is shunted by a Zener diode Z62, which becomesconductive when the emitter-collector voltage exceeds the Zener voltage,the diode thereby determining the maximum emitter-collector voltage. Aresistor R63 and a capacitor C64, connected in parallel, connect thecollector of the transistor T55 to one terminal of the valve MV3, theother valve terminal being connected to the grounded rail 49.

In explaining the circuit shown in FIG. 3, it will be assumed at firstthat the positive feedback resistor R51 is out of circuit. If the samevoltage is present at the inputs M and P, the voltage at the output ofthe operational amplifier 45 is determined by the voltage across theresistors R46 and R47. If the voltage at the summation junction 40rises, thereby raising the voltage at the inverting input M, the voltageat the output of the operational amplifier falls. On the other hand, ifthe voltage at the summation junction becomes less positive, the voltageat the operational amplifier output becomes more positive. It will nowbe assumed that the positive feedback resistor R51 is in circuit. Thevoltage of the inverting input M is now so positive that the operationalamplifier 45 is over driven; consequently, the output voltage of theoperational amplifier is approximately at ground potential. When theoutput voltage is approximately at ground potential, the resistors R47and R51 are connected in parallel, and the ratio between resistors R46and R47 of the voltage divider changes. On the other hand, if thevoltage at the inverting input M is so negative that the operationalamplifier 45 is over driven in the other direction and its outputvoltage is approximately +Ub, the positive feedback resistor R51 isconnected in parallel with the resistor R46 of the voltage divider; andthe voltage at the noninverting input P changes in the positivedirection. Thus, the voltage at the non-inverting input P does notremain constant, but jumps between two limiting values that result fromthe fact that the positive feedback resistor R51 is connected inparallel either with the voltage divider resistor R47 or with thevoltage divider resistor R46, as soon as the output voltage of theoperational amplifier either is approximately at the voltage +Ub or atground potential. For a balanced condition (in which a keying ratio 1 l,for example-- corresponding to the desired value appears at theoperational amplifier output, and in which the deviation is zero) thevoltages at the inputs M and P must be exactly equal, if the positivefeedback resistor R51 is not in circuit.

If the positive feedback resistor R51 is in circuit and the summationjunction 40 is at the corresponding voltage, the electrical output ofthe operational amplifier follows a rectangular wave oscillation, aswill now be explained. It will be assumed that the output voltage of theamplifier 45 has just jumped from ground potential to +Ub. Consequently,the capacitor C65 has just discharged to such a value that the voltageat the input M has fallen below the voltage at the input P. Because ofthe inversion within the operational amplifier 45, this negative voltagedifference between the inverting input and the non-inverting inputcauses the output voltage of the amplifier to become more positive. Thecapacitor C65 now begins to charge through the resistor R50 to apositive voltage. The voltage at which the output voltage of theoperational amplifier jumps back to a ground potential is determined bythe voltage at its non-inverting input; this voltage is determined bythe parallel connection of the resistors R46 and R51, which areconnected in series with the resistor R47, the resistors comprising avoltage divider to which the noninverting input P is connected. Once thevoltage across the capacitor C65 exceeds the new, constant, voltage atthe input P, the positive voltage difference between the two inputs,because of the inversion within the amplifier 45, causes the outputvoltage to jump to approximately ground potential. The positive feedbackresistor R51 is now connected in parallel with the resistor R47, and thecapacitor C65 must discharge to a voltage that is below the voltagedetermined by the parallel connected resistors R47 and R51 before theoperational amplifier is again triggered to cause the output voltage tojump to the other limiting value.

If the charging of the capacitor C65 is altered by a subsidiary additionor withdrawal of current, one of the two limiting states is lengthenedand the other one is shortened. The output voltage of the operationalamplifier is held at one of the two limiting values only when thealteration of the charging is so great that the described oscillationcannot continue, because the voltage at the inverting input M cannotrise above, or fall below, that at the non-inverting input P. The outputamplifier connected between the valve MV3 and the amplifier 45 operatesthe valve in time with the rectangular output voltage of the amplifier45.

FIG. 4 is a schematic diagram of one embodiment of the three pointregulator 32. The regulator has an operational amplifier of which theoutput and the inverting input M are connected in the diagonal of abridge composed of diodes D71 to D74. An adjustable resistor R75connects the input M to the summation junction 36, which is also shownin FIG. 2. The non-inverting input P of the operational amplifier isconnected to the tap of a voltage divider composed of the resistors R76and R77 that are connected between the positive layer 48 and thegrounded layer 49. The operational amplifier 70 is connected to therails 48 and 49 for power. A capacitor C78 dynamically stabilizes theoperational amplifier, which latter is unsymmetrical; the resistor R79is the load resistor of the amplifier. The diodes of the bridge areconnected in forward polarity, the anodes of the diodes D71 and D73being connected by a resistor R80 to the rail 48, and the cathodes ofthe diodes D72 and D74 being connected by a resistor R8l to the rail 49.

The circuit just described operates in the following manner. If thediode bridge is so adjusted that the same voltages are present at theinverting and non-inverting inputs of the operational amplifier, theoutput voltage Ua of the amplifier will be approximately equal to thisvoltage, namely about one-half of the operating voltage plus Ub. If nowthe diode bridge is loaded at the summation junction 46, causing anaddition or withdrawal of current, the symmetry of the bridge isaffected. As soon as the bridge is made sufficiently unsymmetrical sothat the quiescent current in one leg of the bridge stops, the amplifier70 no longer has any feedback, and the output voltage of the amplifiercan assume a positive or negative limiting value depending on whetherthe voltage of the input is positive or negative. The size of the'deadzone dependson the amount of quiescent current in the diode bridge aswell as on the value of the resistor R75. When the quiescent bridgecurrent is stopped, the voltage output of the operational amplifier 70will be negative if the voltage at the inverting input M is morepositive than the constant voltage at the noninverting inputP. If theopposite is true, the output voltage of the amplifier will be positive.The graph of FIG. 4 plots the output voltage Ua of the operationalamplifier 70 against the voltage Ue at the summation junction 36.

It will be understood that each of the elements described above, or twoor more together, may also find a useful application in other types ofcircuits differing from the types described above.

While the invention has been illustrated and described as embodied in acontrol arrangement with a pulse length modulator for a piston, it isnot intended to be limited to the d3tails shown, since variousmodifications and structural changes may be made without departing inany way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can by applying current knowledgereadily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of this inventionand, therefore, such adaptations should and are intended to becomprehended within the meaning and range of equivalence of thefollowing claims.

What is claimed as new and desired to be protected by Letters Patent isset forth in the appended claims:

1. An hydraulic servo system, comprising a positionable member movablein a first direction and alternatively in an opposite second direction;means for selecting a desired position for said member; means fordetermining the actual position of said member; and electrohydrauliccontrol means operative when the difference between the actual anddesired positions is greater than a predetermined value for effectingrapid movement of said member in a single one of said directions towardsthe desired position, and operative when said difference is reduced toless than said value for causing said member to perform an oscillatorymovement composed of longer motions in direction towards the desiredposition alternating with shorter motions in opposite direction so as toeffect a net slower movement of said member to the desired position.

2. A system as defined in claim 1, wherein said member is a hydraulicpiston movable in a hydraulic cylinder, and wherein said control meansincludes a source of pressurized fluid, conduit means for connectingsaid source of fluid and the interior of said cylinder, electri callyactuated first valve means operative when actuated for so controllingthe flow of fluid from said source to the interior of said cylinder asto effect rapid movement of said piston in one or the other of saiddirections, electrically actuated second valve means operative whenactuated for so controlling the flow of fluid from said source to theinterior of said cylinder as to cause said piston to perform saidoscillatory movement, and valve-actuating means for causing said firstvalve means to effect rapid movement of said piston towards the desiredposition when said difference exceeds said value, and for causing saidsecond valve means to cause said piston to perform said oscillatorymovement when said difference is reduced to less than said value.

3. A system as defined in claim 2, wherein said valveactuating meansincludes pulse-length-modulating means for generating a train ofpulse-length-modulated pulses, the ratio of the duration of the pulsesto the time interval between successive pulses varying as a function ofsaid difference when said difference is less than said value, andwherein said valve-actuating means comprises means for so controllingsaid second valve means as to cause said piston to move in one of saiddirections during each of said pulses and in the opposite directionduring the time intervals between successive pulses, so as to cause saidpiston to perform said oscillatory movement.

4. A system as defined in claim 3, wherein as said difference approacheszero said ratio approaches a value causing said piston to oscillateabout the desired position in the steady state.

5. A system as defined in claim 2, wherein said valveactuating meansincludes pulse-length-modulating means for generating a train ofpulse-length-modulated pulses when said difference is less than saidvalue, the ratio of the duration of the pulses to the time intervalbetween successive pulses varying as a function of said difference whensaid difference is less than said value, and wherein saidvalve-actuating means comprises means operative only when saiddifference is less than zero for so controlling said second valve meansas to cause said piston to move in one of said directions during each ofsaid pulses and in the opposite direction during the time intervalsbetween successive pulses, so as to cause said piston to perform saidoscillatory movement.

6. A system as defined in claim 5, wherein as said difference approacheszero said ratio approaches a value causing said piston to oscillateabout the desired position in the steady state.

7. A system as defined in claim 4, wherein said valveactuating means isoperative for causing said first valve means to effect said rapidmovement of said piston towards the desired position only when saiddifference exceeds said value.

8. A system as defined in claim 2; and further comprisingpressure-regulating valve means for regulating the pressure at whichsaid fluid leaves said source.

9. A system as defined in claim 1, wherein said member is a hydraulicpiston movable in a hydraulic cylinder and dividing the cylinder intotwo separate chambers located at axially opposite sides of said piston,and wherein said control means includes a source of pressurized fluid, acollection tank for fluid, a conduit connecting said source to one ofsaid chambers, a further conduit connecting said source to the other ofsaid chambers, an additional conduit connecting said other of saidchambers to said tank, a first shunting conduit shunting said' furtherconduit, a second shunting conduit shunting said additionalconduit,-first valve means comprising a pair of electrically actuatedfirst valves, one disposed in said further conduit and the otherdisposed in said additional conduit, and operative when one or the otheris actuated for effecting movement of said piston in one or the other ofsaid directions, electrically actuated second valve means connected tosaid first shunting conduit and connected to said second shuntingconduit for alternatively blocking flow through one or the other of saidshunting conduits and operative when unblocking said first shuntingconduit for effecting movement of said piston in one direction andoperative when unblocking said second shunting conduit for effectingmovement of said piston in the opposite direction, and valve actuatingmeans operative when said difference exceeds said value for actuatingthe respective one of said first valves to effect said rapid movement ofsaid piston towards the desired position, and operative when saiddifference is reduced to less than said value for actuating said secondvalve means to unblock said first and second shunting conduitsalternately to cause said piston to effect said oscillatory movement.

10. A system as defined in claim 9; and further including a pair ofpressure-regulating valves, one disposed in said first shunting conduitand the other disposed in said second shunting conduit.

11. A system as defined in claim 9, wherein said piston includes apiston rod mounted for sliding movement through an opening in the wallof said cylinder and wherein said rod extends through said one chamber.

12. A system as defined in claim 1 1, wherein said piston rod has across-sectional area substantially equal to the effectivecross-sectional area of the respective axial end face of said piston.

13. A system as defined in claim 3, wherein said pulse-length-modulatingmeans comprises an operational amplifier having an inverting and anon-inverting input, means for maintaining one of said inputs at aconstant potential, a pair of input resistors each connected at one endto the other input of said amplifier, the other end of one of said inputresistors being connected to said means for selecting a position forsaid member, and the other end of the other of said input resistorsbeing connected to said means for determining the actual position ofsaid member, an adjustable capacitor connected between ground and saidother of said inputs of said amplifier, and adjustable feedback resistorconnecting the output of said operational amplifier to said other inputthereof, and a coupling resistor connecting the output of saidoperational amplifier to said one input thereof.

14. A system as defined in claim 7, wherein said valve-actuating meansincludes three-point regulator means comprising an operational amplifierhaving in its input circuit a diode-resistor network for determining thealgebraic sign of said difference and whether said difference is lessthan said value.

15. A system as defined in claim 14, wherein said regulator means has aninput to which is applied the sum of an electrical signal proportionalto the position of said member, an electrical signal proportional to therate of movement of said member and an electrical signal proportional tothe desired position of said member.

16. A system as defined in claim 15, wherein said input of saidregulator means has an input resistor connected to said means fordetermining the actual position of said member, and also adifferentiating capacitor connected in parallel with said inputresistor.

* II! t

1. An hydraulic servo system, comprising a positionable member movablein a first direction and alternatively in an opposite second direction;means for selecting a desired position for said member; means fordetermining the actual position of said member; and electrohydrauliccontrol means operative when the difference between the actual anddesired positions is greater than a predetermined value for effectingrapid movement of said member in a single one of said directions towardsthe desired position, and operative when said difference is reduced toless than said value for causing said member to perform an oscillatorymovement composed of longer motions in direction towards the desiredposition alternating with shorter motions in opposite direction so as toeffect a net slower movement of said member to the desired position. 2.A system as defined in claim 1, wherein said member is a hydraulicpiston movable in a hydraulic cylinder, and wherein said control meansincludes a source of pressurized fluid, conduit meaNs for connectingsaid source of fluid and the interior of said cylinder, electricallyactuated first valve means operative when actuated for so controllingthe flow of fluid from said source to the interior of said cylinder asto effect rapid movement of said piston in one or the other of saiddirections, electrically actuated second valve means operative whenactuated for so controlling the flow of fluid from said source to theinterior of said cylinder as to cause said piston to perform saidoscillatory movement, and valve-actuating means for causing said firstvalve means to effect rapid movement of said piston towards the desiredposition when said difference exceeds said value, and for causing saidsecond valve means to cause said piston to perform said oscillatorymovement when said difference is reduced to less than said value.
 3. Asystem as defined in claim 2, wherein said valve-actuating meansincludes pulse-length-modulating means for generating a train ofpulse-length-modulated pulses, the ratio of the duration of the pulsesto the time interval between successive pulses varying as a function ofsaid difference when said difference is less than said value, andwherein said valve-actuating means comprises means for so controllingsaid second valve means as to cause said piston to move in one of saiddirections during each of said pulses and in the opposite directionduring the time intervals between successive pulses, so as to cause saidpiston to perform said oscillatory movement.
 4. A system as defined inclaim 3, wherein as said difference approaches zero said ratioapproaches a value causing said piston to oscillate about the desiredposition in the steady state.
 5. A system as defined in claim 2, whereinsaid valve-actuating means includes pulse-length-modulating means forgenerating a train of pulse-length-modulated pulses when said differenceis less than said value, the ratio of the duration of the pulses to thetime interval between successive pulses varying as a function of saiddifference when said difference is less than said value, and whereinsaid valve-actuating means comprises means operative only when saiddifference is less than zero for so controlling said second valve meansas to cause said piston to move in one of said directions during each ofsaid pulses and in the opposite direction during the time intervalsbetween successive pulses, so as to cause said piston to perform saidoscillatory movement.
 6. A system as defined in claim 5, wherein as saiddifference approaches zero said ratio approaches a value causing saidpiston to oscillate about the desired position in the steady state.
 7. Asystem as defined in claim 4, wherein said valve-actuating means isoperative for causing said first valve means to effect said rapidmovement of said piston towards the desired position only when saiddifference exceeds said value.
 8. A system as defined in claim 2; andfurther comprising pressure-regulating valve means for regulating thepressure at which said fluid leaves said source.
 9. A system as definedin claim 1, wherein said member is a hydraulic piston movable in ahydraulic cylinder and dividing the cylinder into two separate chamberslocated at axially opposite sides of said piston, and wherein saidcontrol means includes a source of pressurized fluid, a collection tankfor fluid, a conduit connecting said source to one of said chambers, afurther conduit connecting said source to the other of said chambers, anadditional conduit connecting said other of said chambers to said tank,a first shunting conduit shunting said further conduit, a secondshunting conduit shunting said additional conduit, first valve meanscomprising a pair of electrically actuated first valves, one disposed insaid further conduit and the other disposed in said additional conduit,and operative when one or the other is actuated for effecting movementof said piston in one or the other of said directions, electricallyactuated second valve mEans connected to said first shunting conduit andconnected to said second shunting conduit for alternatively blockingflow through one or the other of said shunting conduits and operativewhen unblocking said first shunting conduit for effecting movement ofsaid piston in one direction and operative when unblocking said secondshunting conduit for effecting movement of said piston in the oppositedirection, and valve actuating means operative when said differenceexceeds said value for actuating the respective one of said first valvesto effect said rapid movement of said piston towards the desiredposition, and operative when said difference is reduced to less thansaid value for actuating said second valve means to unblock said firstand second shunting conduits alternately to cause said piston to effectsaid oscillatory movement.
 10. A system as defined in claim 9; andfurther including a pair of pressure-regulating valves, one disposed insaid first shunting conduit and the other disposed in said secondshunting conduit.
 11. A system as defined in claim 9, wherein saidpiston includes a piston rod mounted for sliding movement through anopening in the wall of said cylinder and wherein said rod extendsthrough said one chamber.
 12. A system as defined in claim 11, whereinsaid piston rod has a cross-sectional area substantially equal to theeffective cross-sectional area of the respective axial end face of saidpiston.
 13. A system as defined in claim 3, wherein saidpulse-length-modulating means comprises an operational amplifier havingan inverting and a non-inverting input, means for maintaining one ofsaid inputs at a constant potential, a pair of input resistors eachconnected at one end to the other input of said amplifier, the other endof one of said input resistors being connected to said means forselecting a position for said member, and the other end of the other ofsaid input resistors being connected to said means for determining theactual position of said member, an adjustable capacitor connectedbetween ground and said other of said inputs of said amplifier, andadjustable feedback resistor connecting the output of said operationalamplifier to said other input thereof, and a coupling resistorconnecting the output of said operational amplifier to said one inputthereof.
 14. A system as defined in claim 7, wherein saidvalve-actuating means includes three-point regulator means comprising anoperational amplifier having in its input circuit a diode-resistornetwork for determining the algebraic sign of said difference andwhether said difference is less than said value.
 15. A system as definedin claim 14, wherein said regulator means has an input to which isapplied the sum of an electrical signal proportional to the position ofsaid member, an electrical signal proportional to the rate of movementof said member and an electrical signal proportional to the desiredposition of said member.
 16. A system as defined in claim 15, whereinsaid input of said regulator means has an input resistor connected tosaid means for determining the actual position of said member, and alsoa differentiating capacitor connected in parallel with said inputresistor.